1. Field of the Invention
The present invention relates to a resonator type surface acoustic wave filter used as, for example, a bandpass filter or other type of filter. More specifically, the present invention relates to a resonator type surface acoustic wave filter including a plurality of transversely or longitudinally coupled surface acoustic wave resonators which generates and uses a surface acoustic wave that has a Shear Horizontal (SH) wave as its main component, such as, a Bleustein-Gulyaev-Shimizu (BGS) wave.
2. Description of the Related Art
Various conventional surface acoustic wave filters are known bandpass filters. For example, a transversely coupled surface acoustic wave filter using a Rayleigh wave as a surface acoustic wave, and transversely coupled and longitudinally coupled resonator filters using a Rayleigh wave are already known bandpass filters.
The transverse type surface acoustic wave filter is constructed such that a fixed distance separates a pair of interdigital transducers (IDTs) from each other on a surface acoustic wave substrate.
Further, in the transversely coupled resonator filter using a Rayleigh wave, a plurality of surface acoustic wave resonators include a plurality of IDTs that are provided on a surface acoustic wave substrate with the plurality of surface acoustic wave resonators coupled in the transverse direction. In the transversely coupled resonator filter, reflectors are located on both sides of an area occupied by the IDTs.
Recently, an edge reflection type surface acoustic wave filter using and SH type surface acoustic wave such as a BGS wave has been developed because the filter has the advantage of being compact.
In a transversely coupled surface acoustic wave filter using the Rayleigh wave, because two sets of IDTs are separated by a fixed distance along the direction of the surface-wave transmission on a surface acoustic wave substrate, the size of the filter cannot be reduced and is instead much larger. Also, the insertion loss is very large.
In a transversely coupled resonator filter using a Rayleigh wave, because the electromechanical coupling coefficient K is small, the filter does not have sufficient bandwidth, and although the loss can be reduced, the aperture of the electrode fingers of the IDT is about 10xcex and the coupling length is about 1.5xcex assuming that the wavelength of the surface acoustic wave is B. Additionally, reflectors, as described above, are required. Accordingly, the size of the filter cannot be reduced and made compact.
Further, in an edge reflection type surface acoustic wave resonator using a SH type surface acoustic wave, a resonator with a wide bandwidth is obtained, which is due to the relatively large electromechanical coupling coefficient K, so that no reflectors are required. Further, the edge reflection type resonator is favorable because it can be made compact. However, up until now, when a SH type surface acoustic wave was used, the construction of a filter by coupling the plurality of surface acoustic wave resonators was considered to be impossible.
To overcome the problems of the related art described above, preferred embodiments of the present invention provide a resonator type surface acoustic wave filter constructed so as to make use of a SH type surface acoustic wave, particularly a BGS wave, and which is miniaturized and compact size with very small insertion loss.
A transversely coupled resonator type surface acoustic wave filter according to a preferred embodiment of the present invention includes a surface acoustic wave substrate having opposing first and second end surfaces, first and second surface acoustic wave resonators on the surface acoustic wave substrate, and which utilizes a surface acoustic wave having an SH wave as a main component, and the first and second surface acoustic wave resonators are coupled to constitute a stage of the transversely coupled resonator filter, wherein the first and second surface acoustic wave resonators include first and second interdigital transducers, the first and second interdigital transducers defined by a first bus bar, a second bus bar and a common bus bar all arranged so as to be substantially parallel to each other and extending in a direction of transmission of the surface acoustic wave, the common bus bar is located between the first and second bus bars, and a plurality of electrode fingers are connected to the first and second bus bars at a first end of the electrode fingers and extended towards the common bus bar, a plurality of electrode fingers are connected to the common bus bar at a first end of the electrode fingers and such that a first set of electrode fingers of the common bus bar are extended towards the first bus bar and a second set of electrode fingers are extended towards the second bus bar so that the first and second sets of electrode fingers of the common bus bar interdigitate with the electrode fingers of the first and second bus bars, wherein the surface acoustic wave substrate has a relative dielectric constant E along the direction of 11 of the surface acoustic wave substrate such that E=∈s11/∈0, and where E is in a range of about 0 to about 3000, wherein an aperture y of the interdigital transducer is defined as an amount of overlap between the electrode fingers of the common bus bar with the electrode fingers of the first and second bus bars normalized by a wavelength xcex of the surface acoustic wave, and wherein the filter is constructed such that y satisfies a formula:
0.945+5.49xc3x97exp(xe2x88x92E/366)xe2x89xa7yxe2x89xa72.46xc3x97exp(xe2x88x92E/219)xe2x80x83xe2x80x83(1)
Also, when K is an electromechanical coupling coefficient of the surface acoustic wave substrate, the filter is preferably arranged such that F=K2xc2x7∈s11/∈0, and F is in the range of about 0 to about 250 and the aperture y of the interdigital transducer satisfies the following formula:
1.40+4.14xc3x97exp(xe2x88x92F/46)xe2x89xa7yxe2x89xa70.25+0.97xc3x97exp(xe2x88x92F/42)xe2x80x83xe2x80x83(2)
Further, in a transversely coupled resonator type surface acoustic wave filter, preferably, the coupling length x, which is normalized by the wavelength xcex of the surface acoustic wave, satisfies the following formula:
0.71+1.72xc3x97exp(xe2x88x92E/251)xe2x89xa7xc3x97xe2x89xa70.045+0.16xc3x97exp(xe2x88x92E/418)xe2x80x83xe2x80x83(3)
Preferably, if the filter is constructed so that F=K2xc2x7∈s11/∈0 is in the range of about 0 to about 250 then the coupling length x preferably satisfies the following formula:
0.452+1.953xc3x97exp(xe2x88x92F/49.56)xe2x89xa7xc3x97xe2x89xa70.269xc3x97exp(xe2x88x92F/32)xe2x80x83xe2x80x83(4)
Further, one bus bar of each of the first and second surface acoustic wave resonators is made a common bus bar and the width W of the common bus bar preferably satisfies the following formula:
0.32+0.853xc3x97exp(xe2x88x92E/222)xe2x89xa7Wxe2x89xa70.017+0.157xc3x97exp(xe2x88x92E/245)xe2x80x83xe2x80x83(5)
Also, if the filter is constructed so that K2xc2x7∈s11/∈0 is in the range of about 0 to about 250, then the width W of the common bus bar of the first and second surface acoustic wave resonators preferably satisfies the following formula:
0.22+0.84xc3x97exp(xe2x88x92F/43)xe2x89xa7Wxe2x89xa70.03+0.14xc3x97exp(xe2x88x92F/21)xe2x80x83xe2x80x83(6)
Further, the filter is preferably constructed so that the gap width G between the common bus bar and the tip of a plurality of electrode fingers extending from the other bus bars meets the following formula:
1.19+4.51xc3x9710xe2x88x924xc3x97Exe2x88x921.34xc3x9710xe2x88x926xc3x97E2xe2x89xa7Gxe2x89xa7xe2x88x920.115+0.29xc3x97exp(xe2x88x92E/1150)xe2x80x83xe2x80x83(7)
Also, if the filter is constructed so that F=K2xc2x7∈s11/∈0 is in the range of about 0 to about 250, then the gap width G preferably meets the following formula:
1.125xe2x88x920.003xc3x97Fxe2x88x921.016xc3x9710xe2x88x926xc3x97F2xe2x89xa7Gxe2x89xa7xe2x88x920.107+0.26xc3x97exp(xe2x88x92F/250)xe2x80x83xe2x80x83(8)
Moreover, the outside gap Gxe2x80x2 is also preferably made approximately equal to G.
In a resonator type surface acoustic wave filter, preferably an edge reflection type surface acoustic wave filter that makes use of reflection at the end surface of a substrate is constructed.
Further, the resonator type surface acoustic wave filter of preferred embodiments may include one stage or a plurality of stages.
In another preferred embodiment of the present invention, a longitudinally coupled resonator type surface acoustic wave filter includes a surface acoustic wave substrate, and first and second surface acoustic wave resonators provided on the surface acoustic wave substrate and utilizing a surface acoustic wave having an SH wave as a main component, and the first and second surface acoustic wave resonators are coupled to constitute a stage of the longitudinally coupled resonator filter, wherein the first and second surface acoustic wave resonators include first and second interdigital transducers arranged in the direction of transmission of the surface accustic wave on the surface acoustic wave substrate, respectively, each of the interdligital transducers defined by first and second comb electrodes having a first bus bar and a second bus bar, respectively, and such that the bus bars are extended in the direction of the transmission of the surface acoustic wave, and each of the bus bars, are connected to a first end of a plurality of electrode fingers such that a second end of the electrode fingers of the first bus bar is extended towards the second bus bar and a second end of the electrode fingers of the second bus bar are extended towards the first bus bar such that the electrode fingers of the first and second bus bars interdigitate with each other, wherein the surface acoustic wave substrate has a relative dielectric constant E such that E=∈s11/∈0, and where E is in a range of about 0 to about :3000, wherein an aperture Y is defined as an amount of overlap between the electrode fingers of the first bus bar and the second bus bar normalized by a wavelength xcex of the surface acoustic wave, and wherein Y satisfies a formula:
5.52+66.62xc3x97exp(xe2x88x92E/110)xe2x89xa7Yxe2x89xa70.80+3.48xc3x97exp (xe2x88x92E/404)xe2x80x83xe2x80x83(9)
Also, when K is the electromechanical coupling coefficient of the surface acoustic wave substrate, preferably F=K 2xc2x7∈s11/∈0 is in the range of about 0 to about 250 and the aperture Y preferably satisfies the following formula:
7.96+44.14xc3x97exp(xe2x88x92F/38.3)xe2x89xa7Yxe2x89xa70.40+4.35xc3x97exp(xe2x88x92F/80)xe2x80x83xe2x80x83(10)
Further, the filter is preferably constructed so that the number of pairs N of electrode fingers of the first and second interdigital transducers satisfies the following formula:
114.4+0.02xc3x97Exe2x88x929.283xc3x9710xe2x88x925xc3x97E2xe2x89xa7Nxe2x89xa72.0xe2x80x83xe2x80x83(11)
Alternatively, if the filter is constructed so that F=K2xc2x7∈s11/∈0 is in the range of about 0 to about 250, then the number of pairs of electrode fingers N of the first and second interdigital transducers preferably satisfies the following formula:
xe2x88x92171+292xc3x97exp(xe2x88x92F/672)xe2x89xa7Nxe2x89xa72.0xe2x80x83xe2x80x83(12)
Additionally, the distance X, which is the separation between adjacent electrode fingers of the first and second transducers preferably satisfies the following formula:
xe2x88x925.423+5.994xc3x97exp(xe2x88x92E/22894)xe2x89xa7Xxe2x89xa70.255xe2x88x920.19xc3x97exp(xe2x88x92E/446)xe2x80x83xe2x80x83(13)
Also, if the filter is constructed so that F=K2xc2x7∈s11/∈0 is in the range of about 0 to about 250, then the distance X preferably satisfies the following formula:
0.364+0.198xc3x97exp(xe2x88x92F/67.5)xe2x89xa7Xxe2x89xa70.241xe2x88x920.169xc3x97exp(xe2x88x92F/58.1)xe2x80x83xe2x80x83(14)
Further, a longitudinally coupled resonator type surface acoustic wave filter is preferably constructed as an edge reflection type surface acoustic wave filter that makes use of reflection at a pair of opposing end surfaces of a surface acoustic wave substrate.
In the transversely coupled resonator filter, because the aperture y of the IDT is arranged to be in a certain range and ∈s11/∈0 is in the range of about 0 to about 3000, excellent resonance characteristics are achieved. That is, it is possible to provide a transversely coupled edge reflection type resonator filter that makes use of a BGS wave, although it has been heretofore been thought to be impossible.
Also, because the filter is a resonator type surface acoustic wave filter of edge reflection type, reflectors are not required, and accordingly, it is possible to achieve a small-sized surface acoustic wave filter. Therefore, preferred embodiments of the present invention provide a surface acoustic wave filter having excellent filter characteristics while being compact.
Further, by satisfying the given ranges for the aperture, coupling length, width of the common bus bar, and gap width in accordance with ∈s11/∈0 and K2xc2x7∈s11/∈0, greatly improved filter characteristics are achieved. Also, because the reflectance ratio and reflection efficiency are high compared with those in the reflector of electrode fingers, a low-loss and wideband filter can be realized.
In the longitudinally coupled resonator filter, first and second surface acoustic wave resonators that makes use of a BGS wave is constructed. And as ∈s11/∈0 is in the range of about 0 to 3000 and the aperture Y is made so as to within a certain range, excellent filter characteristics can be obtained.
Particularly, when K2xc2x7∈s11/∈0 is in the range of about 0 to about 250 and the aperture Y is within a certain range, the number of pairs of electrode fingers of the first and second IDTs are within the desired range, and the distance X between adjacent electrode fingers of the first and second IDTs are a desired distance, greatly improved filter characteristics are achieved.
In another preferred embodiment of the present invention, a communication apparatus having a duplexer includes the above described longitudinally coupled surface acoustic wave resonator filter or a transversely coupled surface acoustic wave resonator filter.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.